Single-Slit Diffraction

1. The problem statement, all variables and given/known data
You have been asked to measure the width of a slit in a piece of paper. You mount the paper 80.0 centimeters from a screen and illuminate it from behind with laser light of wavelength 633 nanometers (in air). You mark two of the intensity minima as shown in the figure, and measure the distance between them to be 17.9 millimeters.

[PLAIN]http://img85.imageshack.us/img85/7453/101966.jpg [Broken]

2. Relevant equations

sin([tex]\theta[/tex])=m[tex]\lambda[/tex]/a

Also for small angles [tex]\theta[/tex] sin[tex]\theta[/tex] = tan[tex]\theta[/tex] = [tex]\theta[/tex]

3. The attempt at a solution

I'm not really sure how to set up the equation. I'm used to having questions giving me the width of the central fringe and not the distance between 2 minima.

How do I use the given distance between the paper and screen and the distance between the two minima in the picture to calculate sin[tex]\theta[/tex]